Hydrocarbon-like and oxygenated organic aerosols in Pittsburgh: insights into sources and processes of organic aerosols

[摘要] A recently developed algorithm (Zhang et al., 2005) has been applied to deconvolve the mass spectra oforganic aerosols acquired with the Aerosol Mass Spectrometer (AMS) inPittsburgh during September 2002. The results are used here to characterizethe mass concentrations, size distributions, and mass spectra ofhydrocarbon-like and oxygenated organic aerosol (HOA and OOA, respectively).HOA accounts for 34% of the measured organic aerosol mass and OOAaccounts for 66%. The mass concentrations of HOA demonstrate a prominentdiurnal profile that peaks in the morning during the rush hour and decreaseswith the rise of the boundary layer. The diurnal profile of OOA isrelatively flat and resembles those of SO₄²⁻ and NH₄⁺.The size distribution of HOA shows a distinct ultrafine mode that iscommonly associated with fresh emissions while OOA is generally concentratedin the accumulation mode and appears to be mostly internally mixed with theinorganic ions, such as SO₄²⁻ and NH₄⁺. Theseobservations suggest that HOA is likely primary aerosol from local,combustion-related emissions and that OOA is secondary organic aerosol (SOA)influenced by regional contributions. There is strong evidence of the directcorrespondence of OOA to SOA during an intense new particle formation andgrowth event, when condensational growth of OOA was observed. The fact thatthe OOA mass spectrum from this event is very similar to that from the entirestudy suggests that the majority of OOA in Pittsburgh is likely SOA. O₃appears to be a poor indicator for OOA concentration while SO₄²⁻is a relatively good surrogate for this dataset. Since the diurnal averagesof HOA track those of CO during day time, oxidation/aging of HOA appears tobe very small on the time scale of several hours. Based on extracted massspectra and the likely elemental compositions of major m/z's, the organic massto organic carbon ratios (OM:OC) of HOA and OOA are estimated at 1.2 and 2.2 μg/μgC,respectively, leading to an average OM:OC ratio of 1.8 forsubmicron OA in Pittsburgh during September. The C:O ratio of OOA isestimated at 1:0.8. The carbon contents in HOA and OOA estimated accordinglycorrelate well to primary and secondary organic carbon, respectively,estimated by the OC/EC tracer technique (assuming POC-to-EC ratio=1). Inaddition, the total carbon concentrations estimated from the AMS data agreewell with those measured by the Sunset Laboratory Carbon analyzer (r²=0.87; slope=1.01±0.11). Our results represent the first directestimate of the OM:OC ratio from highly time-resolved chemical compositionmeasurements.

[学科分类] 大气科学